Cardiac hemodynamics and ventricular stiffness of sea-run cherry salmon (Oncorhynchus masou masou) differ critically from those of landlocked masu salmon

Ventricular diastolic mechanical properties are important determinants of cardiac function and are optimized by changes in cardiac structure and physical properties. Oncorhynchus masou masou is an anadromous migratory fish of the Salmonidae family, and several ecological studies on it have been conducted; however, the cardiac functions of the fish are not well known. Therefore, we investigated ventricular diastolic function in landlocked (masu salmon) and sea-run (cherry salmon) types at 29–30 months post fertilization. Pulsed-wave Doppler echocardiography showed that the atrioventricular inflow waveforms of cherry salmon were biphasic with early diastolic filling and atrial contraction, whereas those of masu salmon were monophasic with atrial contraction. In addition, end-diastolic pressure–volume relationship analysis revealed that the dilatability per unit myocardial mass of the ventricle in cherry salmon was significantly suppressed compared to that in masu salmon, suggesting that the ventricle of the cherry salmon was relatively stiffer (relative ventricular stiffness index; p = 0.0263). Contrastingly, the extensibility of cardiomyocytes, characterized by the expression pattern of Connectin isoforms in their ventricles, was similar in both types. Histological analysis showed that the percentage of the collagen accumulation area in the compact layer of cherry salmon increased compared with that of the masu salmon, which may contribute to ventricle stiffness. Although the heart mass of cherry salmon was about 11-fold greater than that of masu salmon, there was no difference in the morphology of the isolated cardiomyocytes, suggesting that the heart of the cherry salmon grows by cardiomyocyte proliferation, but not cell hypertrophy. The cardiac physiological function of the teleosts varies with differences in their developmental processes and life history. Our multidimensional analysis of the O. masou heart may provide a clue to the process by which the heart acquires a biphasic blood-filling pattern, i.e., a ventricular diastolic suction.

The data for each EDPVR were described as an exponential fit based on the following equation: 177 where A, B, and C were constants that describe the ventricular exponential pressure-normalized 184 where v t was the total volume of saline infused into the ventricle at time t, v 0 = 0, and m was 185 the ventricular mass [15].

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Pulsed-wave Doppler echocardiography revealed that the atrioventricular inflow 331 patterns in the masu salmon showed sequential single forward flow waveforms that were 332 synchronized with the P waves, indicating atrial contraction ( Fig 1C). The heart rate of masu 333 salmon as recorded with echocardiography was 109 ± 12.0 beats/min (bpm, N = 5 fish) (S1 334 Table). In masu salmon #5, the time from the R wave to the end of ventricular outflow was

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Interestingly, the forward flow waveforms began to appear in the atrioventricular 345 inflow patterns of the cherry salmon, even before the P waves were recorded ( Fig 1D). When 347 The low forward waveforms appeared before the P wave, and the high forward waveforms 348 appeared after the P wave. Therefore, the former and the latter were presumed to be comparable 349 to the early ventricular diastolic and atrial systolic velocities, respectively. In addition, the 350 sequential monophasic waves in cherry salmon were likely to be the fusion of these two 360 fish) and lower than that of masu salmon (p = 0.0164, S1 Table). 516 on the heart rate ( Fig 1D and 1E, S4 Fig). The inflows observed before detecting the P wave 517 indicated passive ventricular filling, suggesting that the ventricle in cherry salmon acquired the 518 ability to suction blood from the atrium. However, the atrial systolic velocity was greater than 519 the early diastolic velocity (Fig 1F, E/A ratio = 0.48 ± 0.06); therefore, it was clear that atrial 520 contraction plays a dominant role in ventricular filling in cherry salmon, at least under 521 anesthesia.

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A negative correlation has been reported between body mass and resting heart rate in 523 both mammals and birds [91, 92], but this rule does not always apply to fish [93]. However, in 524 our study, at 29-30 mpf, cherry salmon was over 10 times heavier than masu salmon (Table   525 1), and the heart rate of masu salmon was placed higher than that of cherry salmon (S1 Table, 526 p = 0.0164). A previous hemodynamic study of rainbow trout had suggested that the heart rate 24 527 is associated with the appearance of monophasic or biphasic atrioventricular inflow waveforms  553 its application in comparing the stiffness of ventricles of different sizes is difficult. In a smaller 554 ventricle, the ratio of the pressure increase relative to the volume increase is greater than for a 555 larger one. Thus, the EDPVR curve for a smaller heart would be steeper than that for a larger 556 ventricle, even with the same actual stiffness. In this study, to evaluate the relative ventricular 557 stiffness of masu and cherry salmon, we calculated the stiffness per unit of myocardial mass 558 by normalizing the ventricular end-diastolic volume by the ventricular mass [15]. The index of 559 ventricular stiffness per unit myocardial mass of cherry salmon, calculated from the normalized 560 EDPVR curve (Fig 2A), was significantly higher than that of masu salmon (Fig 2B), thus 561 suggesting that the ventricle of cherry salmon has a relatively suppressed diastolic function.

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The ventricular walls of cherry salmon were thickened, and its coronary vascular 563 network was well developed, as compared to those of masu salmon (Fig 3D and 3H). According 564 to Laplace's law, the thicker the ventricular wall, the greater the wall tension generated in 565 response to increased internal pressure, and the greater the ventricular stiffness. Physiological 566 hypertrophy in the ventricular wall, which causes both stroke volume and systolic pressure to 567 increase, improved the overall cardiac output [99, 100]. Therefore, despite the expected high 568 performance of cherry salmon hearts, their ventricles were stiffer than those of masu salmon 569 (Fig 2). Coronary circulation presents hemodynamic characteristics that are not observed in

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The compact layer of cherry salmon also contained more collagen fibers as compared 589 with masu salmon (Fig 3E, 3I,